According to an energy review material by a US National Laboratory NREL, the energy sources, which are currently and mainly used, are petroleum, coal, and gas. These energy sources amount to 80% of the energy source which is totally used. However, the current depletion state of petroleum and coal energy is gradually becoming a big problem, and emissions of increasing carbon dioxide and other greenhouse gases into the atmosphere have been generating severe problems. In contrast, the use of renewable energy which is non-polluting green energy is only about 2% of the total energy source. Thus, worries about solving the problems of the energy source have become a motivational reason for promoting studies to develop new renewable energy. Among the new renewable energy sources such as wind, water, and the sun, solar energy is drawing the most attention. Solar cells using solar energy produce less pollution, are limitless in terms of resources, have a semi-permanent lifetime, and thus are expected as an energy source capable of solving the future energy problems.
A solar cell is a device which may directly convert solar energy into electric energy by applying a photovoltaic effect. A solar cell may be divided into an inorganic solar cell and an organic solar cell, depending on the materials constituting a thin film. Typical solar cells are made through a p-n junction by doping crystalline silicon (Si), which is an inorganic semiconductor. Electrons and holes generated by absorbing light diffuse to p-n junction points and move to an electrode while being accelerated by the electric field. The power conversion efficiency in this process is defined as the ratio of electric power given to an external circuit and solar power entering the solar cell, and the efficiency have reached approximately 24% when measured under a currently standardized virtual solar irradiation condition. However, since inorganic solar cells in the related art have already shown the limitation in economic feasibility and material demands and supplies, an organic solar cell, which is easily processed and inexpensive and has various functionalities, has come into the spotlight as a long-term alternative energy source.
For the early organic solar cell, the group led by Professor Heeger at UCSB in the US initiatively led the development of technology. The organic solar cell has an advantage in that monomolecular organic materials or polymer materials used easily and quickly enable an inexpensive and large area process.
However, according to the studies, which have been conducted to date, the organic solar cell has a disadvantage in that the energy conversion efficiency is low. Therefore, in order to secure the competitiveness with other solar cells at this point, it can be said that an improvement in efficiency is very important.